This invention relates to extraction of alumina from aluminous material of high-silica content. More particularly, this invention relates to the processing of sodium aluminate crystals obtained from a concentrated caustic-lime digest of such aluminous materials.
The recovery of alumina from aluminous materials of high-silica content such as, for example, anorthosite, clays, shale, Bayer red mud, and the like has been successfully carried out using a digest of lime and highly concentrated caustic. However, this digest results in the formation of an aluminate solution having a very high caustic concentration and a low alumina to soda ratio. Evaporation of this digest slurry results in the crystallization of sodium aluminate having the formula Na.sub.2 Al.sub.2 O.sub.4.2.5H.sub.2 O. These crystals can then be redissolved, for example, in water or dilute caustic to provide a solution having a more acceptable alumina to soda ratio as well as a lower total caustic content from which aluminum hydroxide can be recovered by conventional means, such as the Bayer process.
Such crystals, after centrifugation or filtration, retain mother liquor having an unacceptably high caustic content. While subsequent washing of the crystals in pure water would act to lower the concentration of the caustic, it would also result in dissolution of the crystals. Since the wash liquid would be recycled to recover the caustic, such dissolution would not be desirable in view of the need to subsequently remove such added water by evaporation in other portions of the process. Ponomarev et al, in their article "The Hydro Chemical Alkali Method of Processing Nepheline Rocks" (pages 45-51 in Tavetnye Metally, 1957), suggest that one separates the crystals from the mother liquor by "pressing or wringing out" the liquor from the crystals. They state that by usual filtration there remains on the filter a deposit with a 1.4 to 1.8 caustic ratio (which, translated to conventional U.S. alumina to caustic expressed-as-carbonate terminology, would be 0.687 to 0.534). The same individuals in Russian Patent 108,917 state that the crystallized alkali aluminate is separated from the mother liquor without any further explanation as to how this separation is to be carried out and to what extent it is effective.
It has now been discovered that most of the high caustic mother liquor may be separated from the sodium aluminate crystals by washing while minimizing dissolution of the crystals and dilution of the liquid in the system.